The purpose of the proposed research is to measure the in vivo contact forces active across a total hip prosthesis. This will be accomplished by implanting a series of instrumented, telemeterized hip prostheses which can measure three components of force acting across the joint. The prosthesis which will be used has been developed in previous work at Case Western Reserve University. Data will be obtained at regular intervals over a minimum of one year post surgery for six patients. Data will be obtaining during a variety of activities such as walking, single and double stance, rising from sitting in a chair, as well as others. To obtain a direct comparison of measured forces versus those predicted via gait analysis and mathematical modeling techniques, a series of combined gait analysis and force data acquisition sessions will be carried out. The results will provide for the first time a direct evaluation of the accuracy of the predictive models based on optimization methods for muscle and joint forces. In addition to addressing the question of the validity and accuracy of the force prediction models, the data will be directly useful in defining loading information for design and analysis of hip joint prostheses, and patient care relative to activities of daily living.